The present application relates to a cathode active material, a cathode, a nonaqueous electrolyte battery, and a method for manufacturing a cathode. More specifically, it relates to a cathode active material containing a lithium composite oxide, a cathode, a nonaqueous electrolyte battery, and a method for manufacturing a cathode for nonaqueous electrolytic solution secondary batteries.
In recent years, a portable equipment, such as a video camera and a notebook computer is widely used, and there is a strong demand for a secondary battery having a small size and a high capacity. An example of the secondary battery currently in use is a nickel-cadmium battery formed by using an alkali electrolytic solution. The battery voltage is as low as about 1.2 V, and thus it is difficult to improve the energy density. Therefore, a lithium metal secondary battery using lithium metal having a specific gravity of 0.534 which is the lightest among the simple substances of solids, an extremely base potential, and the highest current capacity per unit mass among metal anode materials has been developed.
However, as for the secondary battery formed using lithium metal as an anode, when the lithium metal secondary battery is charged and discharged, lithium is grown into a dendrite form in the anode, which cause problems such as the deterioration in the cycle characteristics of the secondary battery and the occurrence of an internal short circuit due to lithium penetrating through a separator that is arranged so that the cathode is not contact with the anode.
Then, for example, as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 62-90863, a secondary battery in which a carbonaceous material such as coke is used for an anode, and charge and discharge cycles are repeated by doping or de-doping an alkali metal ion has been proposed. Thus, it is found that the defect of the deterioration of the anode caused by repeating charge and discharge cycles can be avoided by using the secondary battery.
On the other hand, as a cathode active material capable of obtaining a battery voltage of approximately 4 V, inorganic compounds such as transition metal oxides including alkali metal and transition metal chalcogens are known. Among them, a lithium composite oxide such as lithium cobalt oxide or lithium nickel oxide holds great promise in terms of a high potential, stability and long life.
Among them, a high-nickel cathode active material typified by LixNiO2 is a lithium composite oxide which contains the highest proportion of nickel Ni among constituent metal elements except lithium. The cathode active material shows a higher discharging capacity as compared to LixCoO2 and is an attractive cathode material.
However, larger amounts of LiOH which is a residue of a cathode raw material (an impurity) as well as Li2CO3 which is produced by carbon dioxide gas absorption by LiOH in the air are present on the surface of the high-nickel cathode active material as compared to that of LixCoO2.
Since LiOH in the impurities is an alkali component, when the cathode active material is kneaded with polyvinylidene fluoride (PVdF) to be used as a binder and N-methyl-2-pyrrolidone (NMP) in a step of fabricating a cathode, or when the solvent is applied after the kneading, the gelation of the solvent is caused.
Li2CO3 in the impurities is hardly dissolved in the solvent or the electrolytic solution, however, it is decomposed by the charging and discharging operation, thereby generating gases CO2 and CO3. These gas components increase the pressure inside the battery and lead to the expansion of the battery as well as the deterioration of cycle life. In the case where an exterior member of the battery is made of a stainless steel (SUS) can or an aluminum can and has a high strength, the battery can be damaged by the increased internal pressure due to the generation of gas.
As a method for preventing the gelation, there is a method for neutralizing the alkali component so as to be Li2CO3 by once storing the high-nickel cathode active material in carbon dioxide gas. However, the pH of the cathode active material after the neutralization is higher than that of LixCoO2. Therefore, the decomposition of the electrolytic solution is facilitated and gases CO2 and CO3 are generated.
Thus, as another method for preventing the gelation, a method for preventing the gelation as well as inhibiting the generation of gas is disclosed in JP-A No. 2006-286240. In this method, the residual LiOH is fixed as LiF by treating the cathode active material with a fluorine gas. Therefore, the gelation can be prevented and the generation of gas can be inhibited.
In addition to the above-described problems, there are further problems that the volume density of the electrode is low due to the composition and shape of the high-nickel cathode active material and the winding characteristics of an electrode are poor.
In order to compare a typical shape of LixCoO2 with a shape of LixNiO2, electron microscope images of one example of LixCoO2 and one example of LixNiO2 are shown in FIGS. 1A and 1B. FIG. 1A shows an electron microscope image of an example of LixCoO2. FIG. 1B shows an electron microscope image of an example of LixNO2. In the high-nickel cathode active material, a true specific gravity of the powder is lower as compared to that of LixCoO2, and thus it may be impossible to improve the reduction in the volume density of the electrode by the composition.
Further, a battery with a cylindrical shape can be produced by using the high-nickel cathode active material. In the case where a battery with a flat type, used for portable telephones, is produced, a curve is tight at the time of folding the electrode because the winding characteristics of an electrode are poor. Furthermore, the electrode may be broken or cut at the time of folding the electrode by winding or at the time of molding by pressing after the winding, and therefore it is difficult to produce the battery with a flat type.
In related art, a method for improving the strength by increasing the thickness of electrode foil-shaped or a method for reducing the volume density of the cathode active material applied to the electrode foil has been proposed as a method for reducing the cracking and cutting of the electrode by winding and pressing.
The method of fluorination treatment proposed in JP-A No. 2006-286240 has the following problems (1) to (3):
(1) fluorine gas is highly toxic and difficult to handle;
(2) the internal resistance of the battery is increased by LiF produced as a by-product material and thus the capacity is decreased, and further the capacity is decreased by the corrosion due to the fluorine gas in the cathode active material; and further (3) the residual F is easily reacted with minute amounts of moisture present in the active material and the electrolytic solution to generate HF, thereby causing the cycle deterioration.
With reference to the problems of the volume density of the electrode and the winding characteristics of the electrode, when the above-described method in related art is used, the amount of the cathode active material relative to the volume of the battery is reduced. As a result, it may be impossible to obtain a sufficient capacity. Further, even if it can be wound, it is difficult to mold by pressing. The battery at a laboratory level can be fabricated by using a method for winding while the electrolytic solution is applied to the electrode or a method including the steps of winding, impregnating with the electrolytic solution, molding, and removing the excessive electrolytic solution in place of a method for molding by pressing. However, there are some problems that the composition of the electrolytic solution to be produced and the amount of the electrolytic solution become unclear.